The present invention relates to a radiometer and, more particularly, to a multiband image mapping radiometer mounted on a spacecraft, aircraft or similar craft flying over an object zone to be imaged, e.g., the ground surface of the earth.
A radiometer of the type described is used for remote sensing an object zone extending along the flight path of a craft. Typical of such a radiometer is a multi-spectral scanner (MSS) or a thematic mapper (TM) mounted on LANDSAT which is one of spacecrafts For remote sensing developed by NASA (National Aeronautics and Space Administration).
Recently proposed are a multi-spectral selfscanning radiometer (MESSR) as disclosed in a paper entitled "THE DEVELOPMENT OF MULTISPECTRAL SELF-SCANNING RADIOMETER FOR MOS-1", PROCEEDINGS OF THE FOURTEENTH INTERNATIONAL SYMPOSIUM ON SPACE TECHNOLOGY AND SCIENCE issued in Tokyo in 1984,, pp. 1313-1319, and a high resolution visible camera (HRV) as disclosed in a paper IAF-83-109 entitled "The SPOT-HRVI Instrument: An overview of Design and Performance" and presented at the 34th CONGRESS OF THE INTERNATIONAL ASTRONAUTICAL FEDERATION held in Budapest in 1983. The ESSR AND HRV are image mapping radio meters of the type using a linear CCD (Charge Coupled Device) array as a photoelectric transducer.
The image mapping radiometers described above each measures the intensity of visible rays or infrared rays radiated or reflected by the object zone of the ground surface, i.e., the radiance and generates an image made up of pixels of lightness matching the measured radiance. Generally, this kind of radiometer is provided with a multi-spectral band, or simply multiband, construction which divides the visible rays or infrared rays into a plurality of wavelength bands, i.e., spectral bands and generates an image in each of the bands. A camera section is included in the radiometer and constituted by optics for converging the radiation or reflection from the object zone into a radiant flux corresponding to the radiance, and a detector including a photoelectric tube for converting the radiant flux to an electric signal and a linear CCD array or similar photoelectric device. The radiant flux is divided into the above-mentioned plurality of bands by split optics located on the optical path and implemented by a band separation mirror or a prism. Detectors each being assigned to one of the bands are located at the focal points of the respective bands for converting the incident divided fluxes to corresponding electric signals. The MSSR using linear CCD arrays as the detectors has three bands for visible rays and one band for infrared rays, i.e., four spectral bands in total. Therefore, the radiant flux is divided into four and detected by four detectors. When the detectors assigned to the respective bands are deviated in position from one another or when the optical paths defined by optics assigned one-to-one to the detectors are not identical with designed ones, the received images of the associated bands are deviated from one another with respect to the positions of the pixels. Matching the positions of the pixels is generally referred to as registration. Also, matching the positions of the multiband images is called band-to-band registration.
In the MSS, TM or similar image mapping radiometer of mechanical scanning type using oscillation or rotatable mirror, only several or slightly more than ten photoelectric transducer elements suffice for each band, allowing the band-to-band registration to be readily set up. Specifically the MSS assigns six photoelectric transducer elements to each band while the TM assigns sixteen photoelectric transducer elements to each band (four elements to one of the bands). Further, the pitch of the transducer elements corresponding to the pixel pitch is as great as about 100 microns. Therefore, the allowable alignment error of transducer elements in The event of assembly of the detectors is as great as about one-tenth of the pixel pitch. In addition, the band-to-band registration changes little despite the shock ascribable to launching, temperature changes and other external factors.
On the other hand, the MBSSR, HRV or similar multiband image mapping radiometer includes linear CCD array as detectors. With this type of radiometer, it is necessary to effect the registration with each of the plurality of CCD elements of each CCD array assigned to a particular band. Specifically, each linear CCD array has 2,048 CCD elements, an element pitch of 14 microns, and a focal plane which is 28.67 millimeters long. Hence, assuming that the allowable band-to-band registration error is within 0.1 pixel as with the MSS, then the maximum allowable error of deviation between the associated elements of the different bands in the lengthwise direction is 1.4 microns. Regarding a multiband radiometer of electronic scanning type under development and having about 5,000 CCD elements, the pixel pitch is about 7 microns and, therefore, the maximum allowable band-to-band registration error is 0.7 microns. Confining the error in such a range by the state of the art technologies is extremely difficult. More specifically, with the MESSR or HRV, it is difficult to set up the band-to-band registration although it has the essential advantage of electronic scanning that a dwell time, i.e., a period of time for observing a target is long for the same resolution and the same swath width and, therefore, the optics can be miniaturized and the structure is free from movable portions and, therefore, simple.
Remote sensing using the multiband radiometer not only processes a plurality of bands of images separately but also extracts characteristics by comparing the characteristics of the band-by-band images of the object zone. For this reason, a multiband image which is the combination of the plurality of bands of images is often used.
On the other hand, the conventional radiometers of the type concerned lack a mechanism for detecting an adjustment error which may occur in the band-to-band registration. It has been customary, therefore, to adjust or correct the band-to-band registration when the radiometer is produced or when it is mounted on a craft. Moreover, with any of the conventional radiometers, a deviation of band-to-band registration ascribable to the deformation of structural bodies and detector mounting portions due to the shock of launching, temperature changes and so forth cannot be detected once the craft is launched. Geometric correction to be effected on the earth and using a particular target position on an actual image as a reference has been the only implementation available for the band-to-band registration after launching. Specifically, geometric correction selects several target points on an image of given band, i.e., the tips of peninsulas or similar characteristic points on the earth having noticeable differences in output level, and measures, by using the pixel positions of the output image and time as a reference, the deviations of the target points on the images of the other bands on a pixel basis, thereby correcting the image processing parameters. The geometric correction scheme, however, cannot cope with the number of pixels and the amount of image processing which are increasing to meet the demand for higher resolutions and broader swath widths. More specifically, the amount of information to be processed for the geometric correction is increased due to the increase in resolution and swath width, obstructing the miniaturization of the image processing unit and high speed image processing. In addition, the selection of the characteristic points on the earth is limited by weather, lowering the geometric correction accuracy.